1. Field of the Invention
The present invention relates to a brake apparatus for a vehicle, particularly relates to an apparatus for pressing a friction member onto a brake member by means of rotating force of a vehicle wheel, to restrain the wheel from rotating.
2. Description of the Related Arts
As for a brake apparatus for a vehicle, it has been proposed that a friction member is pressed onto a brake member by means of an electric motor to restrain a wheel from rotating. In Japanese Patent Laid-open Publication No. 2003-287069, for example, there is disclosed a motor driven brake apparatus which converts a rotation of an electric motor into a thrust motion for driving a piston, to press a friction pad onto a disc rotor, thereby to produce braking force. In the above Publication, it has been proposed that a controller for controlling the electric motor is integrally installed in a disc brake device, and that a wire harness for a wheel speed sensor is connected to the controller.
With respect to a conventional disc brake apparatus actuated by hydraulic pressure, it has been proposed to save energy of a power source by employing a piston pump, instead of a prior master cylinder or an electrically actuated pump. The piston pump is disposed in the vicinity of a disc rotor or disc brake device, to convert the rotating force of the wheel into a driving force for the piston pump, which generates hydraulic pressure for actuating a disc brake piston to apply pressing force to a friction member, thereby to apply the braking force to the wheel.
For example, there is disclosed a brake actuator with electromagnet, cam and hydraulic piston components in Japanese Patent Laid-open publication No. 6-17856, which corresponds to the U.S. Pat. No. 5,388,669. The brake actuator includes an additional disc (identified as a second coupling half) provided in parallel with a brake disc on a common axis to be rotated independently of the disc, and connects or disconnects the discs by means of an electromagnetic clutch, which is capable of controlling the pressing force applied to the discs in response to electric current fed to the clutch. The additional disc is provided with a cam race on its circumferential surface, with a radius of the cam being varied along the circumference. The piston is in contact with the cam race, and generates the hydraulic pressure in response to rotation of the disc, to act as the piston pump. The friction power of the clutch is set by properly adjusting the electric current fed to the electromagnetic clutch, whereby the rotating torque of the cam disc and the driving force of the piston for the piston pump associated with the cam disc are set, so that the cylinder pressure, and therefore the braking force can be adjusted.
According to the brake actuator as described in the Japanese Patent Laid-open Publication No. 2003-287069, however, the braking force is produced with the friction pad being pressed onto the disc rotor in response to rotation of the electric motor, a large consumption of electric power is required comparing with prior brake apparatuses, and a countermeasure is required for obtaining the braking force, after supply of the electric power to the motor is terminated. Therefore, the apparatus shall be necessarily high in price.
On the other hand, according to the brake actuator as described in the U.S. Pat. No. 5,388,669, the driving energy is drawn only from the rotating force of the wheel, the braking force can not be applied when the vehicle stops. In addition, the brake actuator requires the electromagnetic clutch, piston pump and cum mechanism, and practically further requires a reservoir tank and many other components. Consequently, the brake actuator is not practical in view of an installation space, weight, cost and productivity, so that it is difficult to put it on a market in fact. As the accuracy for adjusting the braking force depends on the accuracy for transmitting the torque of friction clutch, the braking control shall result in being unstable and inaccurate, in general. Therefore, it is very difficult to achieve such an accurate control as required by the present braking system. Also, as brake fluid has been filled between the piston pump and the cylinder provided for the disc brake device, cubic variation due to temperature difference or the like can not be absorbed. Therefore, it is difficult to provide a desired braking characteristic.
Furthermore, as for a reduction mechanism for providing large reduction ratio, a Ferguson Mechanical Paradox has been known heretofore. For example, on pages 164-165 of a book entitled “Kikougaku”, First edition, 7th print, written by K. Ogawa and I. Katoh and published on Mar. 1, 1976 by Morikita Shuppan Co. Ltd., it is explained that a pair of sun gears with different number of inner theeth, i.e., stationary sun gear and rotating sun gear, are engaged with a common planetary gear.